The large larval sea lamprey (4-5 years old) offers a unique opportunity to study the mechanisms of axonal regeneration in a vertebrate CNS. It recovers behaviorally form complete spinal transection due to axonal regeneration across the zone of injury. Specifically has been demonstrated in both the direction of neurite regeneration and in the selection of postsynaptic targets. We now propose to determine the structural components of the supporting environment through which these axons grow and which may contribute to the specificity of regeneration. Preliminary data suggest that glial elements are closely apposed to control and regenerating Muller axons and also surround their growth cones, suggesting that growth cones advance selectively along glial structures during regeneration. This hypothesis will be tested by examining the structures through which extending axons grow at both the light and EM levels. The EM analysis will focus particularly on the local environment at points of deviation of the regenerating axons and on the growth cone and filopodia. However, we have also noted occasional presynaptic terminals contacting the growth cone and filopodia. Because afferent synaptic activity may modulate axon elongation, we will determine the distribution frequency of such synapses and attempt to identify the cells of origin. The scar in the lamprey, unlike that in mammals, does not appear to be an impediment to regeneration. Whether the scar provides active support for regeneration will be determined in animals with hemisected cords, giving axon a choice to grow preferentially through the scar in their normal direction or grow around it. The cells of origin of the scar will also be determined using 3H- Thymidine to label cells autoradiographically. Cuprolinic blue staining for proteoglycans in light and electron microscopy and immunohistochemical staining for laminin and fibronectin will be used to determine if an extracellular matrix is present at any time during scar formation.